Lung cancer is a disease that begins in the cells of the lungs. In general, there are two main categories of lung cancer: non-small cell lung cancer and small cell lung cancer. Non-small cell lung cancer may be treated using surgery, radiation, and/or chemotherapy. Because lung cancer varies from person to person, no single treatment may be effective for all patients. Typical surgeries for treating lung cancer include lobectomy (removing an entire lobe of a lung), pneumonectomy (removing an entire lung), and wedge or segmental resection (removing a small part of a lung). Surgery is generally not used if the cancer has spread to both lungs, other structures in the chest, the lymph nodes, or other organs. Surgery is also not used to treat tumors at central locations of the lung in which removal is not possible or in the case of small cell lung cancers. Surgery, therefore, is not a viable option for many patients. Surgical treatments may also result in complications with anesthesia or infection, and surgical treatments may have long, painful recovery periods.
Radiation therapy has become a significant and highly successful process for treating lung cancer, brain cancer and many other types of localized cancers. Radiation therapy is particularly useful for treating centrally located tumors and/or small cell tumors that cannot be removed surgically. Radiation therapy can be used as a curative treatment or as a palliative treatment when a cure is not possible. Additionally, surgery and chemotherapy can be used in combination with radiation therapy.
Radiation therapy procedures generally involve (a) a planning process to determine the parameters of the radiation (e.g., dose, shape, etc.), (b) a patient set-up process to position the target at a desired location relative to the radiation beam, (c) radiation sessions to irradiate the cancer, and (d) qualification processes to assess the efficacy of the radiation sessions. Many radiation therapy procedures have several radiation sessions over a period of approximately 5-45 days. Recent advances in radiation therapy, such as three-dimensional conformal external beam radiation, intensity modulated radiation therapy (IMRT), stereotactic radiosurgery and brachytherapy, provide effective treatments for cancer. These newer treatment modalities are often more effective than previous radiation therapies because they can deliver very high doses of radiation to the tumor.
To further improve the treatment of localized cancers with radiotherapy, it would be desirable to increase the radiation dose because higher doses are more effective at destroying most cancers. Increasing the radiation dose, however, also increases the potential for complications to healthy tissues. The efficacy of radiation therapy accordingly depends on both the total dose of radiation delivered to the tumor and the dose of radiation delivered to normal tissue adjacent to the tumor. To protect the normal tissue adjacent to the tumor, the radiation should be prescribed to a tight treatment margin around the target to avoid irradiating healthy tissue. For example, the treatment margin for lung cancer should be selected to avoid irradiating healthy lung tissue. Therefore, it is not only desirable to increase the radiation dose delivered to the tumor, but it also desirable to mitigate the volume of healthy tissue subject to radiation and the dose of radiation delivered to such healthy tissue.
One difficulty of radiation therapy is compensating for movement of the target within the patient either during or between radiation sessions. This is particularly true in the case of central tumors. For example, tumors in the lungs move significant distances during radiation sessions because of respiration and cardiac functions (e.g., heartbeats and vasculature constriction/expansion). To compensate for such movement, the treatment margins are generally larger than desired so that the tumor does not move out of the treatment volume. This is not a desirable solution because the larger treatment margins may cause more normal tissue to be irradiated.
Another challenge in radiation therapy is accurately aligning the tumor with the isocenter of the radiation beam. Current setup procedures generally align external reference markings on the patient with visual alignment guides for the radiation delivery device. For an example, a tumor is first identified within the patient using an imaging system (e.g., X-ray, computerized tomography (CT), magnetic resonance imaging (MRI), or ultrasound system), and then the approximate location of a tumor in the body is aligned with two or more alignment points on the exterior of the patient. During setup, the external marks are aligned with a reference frame of the radiation delivery device to position the treatment target within the patient at the beam isocenter of the radiation beam (also referenced herein as the machine isocenter). Conventional setup procedures using external marks are generally inadequate because the target may move relative to the external marks between the patient planning procedure and the treatment session and/or during the treatment session. As such, the target may be offset from the machine isocenter even when the external marks are at their predetermined locations for positioning the target at the machine isocenter. Reducing or eliminating such an offset is desirable because any initial misalignment between the target and the radiation beam will cause normal tissue to be irradiated. Moreover, if the target moves during treatment because of respiration or cardiac functions, any initial misalignment will likely further exacerbate irradiation of normal tissue. Thus, the day-by-day and moment-by-moment changes in radiation treatment setup and target motion have posed significant challenges for increasing the radiation dose applied to patients.
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